System and method for producing artwork for printed circuit boards

ABSTRACT

System and method utilizing a laser beam to produce artwork for printed circuit boards. The laser beam is modulated in accordance with the artwork and scanned across an output medium to form an image of the artwork. Compensation is provided for chromatic aberration and non-linearities which would otherwise detract from the quality and accuracy of the image.

This is a division, of application Ser. No. 042,133 filed May 24, 1979.

This invention pertains generally to the manufacture of printed circuitboards and more particularly to a system and method for producingartwork for printed circuit boards.

In the manufacture of printed circuit boards, artwork masters arecommonly employed in the formation of conductive lead patterns on theboards. The lead patterns can include a large number of relativelynarrow leads spaced closely together. These leads must be sharplydefined, and an artwork master having high resolution and accuracy isrequired. Heretofore, artwork masters have been preparedphotographically, and laser systems of the prior art have generally notbeen capable of the high degree of accuracy required in the productionof such artwork.

It is in general an object of the invention to provide a new andimproved system and method utilizing a laser beam for producing artworkfor printed circuit boards.

Another object of the invention is to provide a system and method of theabove character which can produce more precise artwork than has beenpossible with laser techniques of the prior art.

Another object of the invention is to provide a system and method of theabove character in which a precise relationship is maintained betweenthe input data and the output image notwithstanding chromatic aberrationand other image distortions in certain elements of the system.

These and other objects are achieved in accordance with the invention byproviding a system and method wherein a laser beam is modulated inaccordance with data defining the artwork to be produced. The modulatedbeam is directed toward an output medium and deflected to effectscanning of the medium to form an image of the artwork on the medium. Areference beam is deflected with the laser beam and directed toward areference mask to provide a reference signal which defines the positionof the laser beam and is employed to provide synchronization between themodulation of the laser beam and the position of the laser beam on theoutput medium. Compensation is provided to maintain a preciserelationship between the input data and the output image notwithstandingnon-linearities, chromatic aberration and other image distortionsintroduced by certain elements of the system.

FIG. 1 is an isometric view, partly broken away and somewhat schematic,of one embodiment of a system incorporating the invention for producingprinted circuit artwork.

FIG. 2 is an enlarged fragmentary elevational view taken along line 2--2in FIG. 1.

FIG. 3 is a fragmentary plan view taken along line 3--3 in FIG. 2.

FIG. 4 is a fragmentary side elevational view taken along line 4--4 inFIG. 2.

FIG. 5 is an enlarged fragmentary sectional view taken along line 5--5of FIG. 3.

FIG. 6 is an enlarged fragmentary sectional view taken along line 6--6in FIG. 3.

FIG. 7 is a functional block diagram of the system of FIG. 1.

FIG. 8 is a plan view, somewhat schematic, of the reticle of thereference mask in the system of FIG. 1.

FIG. 9 is a block diagram of the reference clock generator and imagecorrection circuits of the system of FIG. 7.

FIG. 10 is a block diagram of a carriage and scanner synchronizingcircuit for the system of FIG. 1.

FIGS. 11 and 12 illustrate certain image distortions which are correctedin the system of FIG. 1.

FIG. 13 is a block diagram of a circuit for compensating fornon-linearities of the type illustrated in FIGS. 11 and 12.

FIG. 14 is a timing diagram for the system of FIG. 7.

As a general overview, data defining the artwork to be produced isstored on magnetic tape and read into the memory of a computer. An imageof the artwork is formed on an output medium scanned in raster fashionby a laser beam modulated in accordance with the data from the computer.In the preferred embodiment, the output image has a size of 17.8"×24.2",with 1,000 scan lines per inch and 1,000 pixels per inch in each scanline. Thus, every 0.001 inch in either scan direction over the entireimage area is addressable. Suitable output media include diazo film,silver halide film, and glass plates. With a suitable laser, it may alsobe possible to form the image directly on the printed circuit boards.

The entire system is enclosed in a cabinet 21 having a first section 22which houses the computer, tape deck, power supply and other electroniccomponents, with doors 23 providing access to these components. Thecabinet also includes a second section 24 which encloses the laser,optical, scanning and carriage assemblies of the system, with suitableaccess doors (not shown). A keyboard terminal 26 provides communicationwith the computer and is mounted on a shelf 27 which projects fromcabinet section 22 so that the terminal is accessible externally of thecabinet.

The laser, optical, scanning and carriage assemblies are supported by amassive table 29 to reduce vibration and thermally induced variationsand permit the accuracy required for printed circuit artwork. In thepreferred embodiment, the table is made of granite and includes ahorizontally extending slab 31 having a thickness of approximately 8-10inches supported by pedestals 32 and leveling blocks 33. The uppersurface 34 of slab 31 is highly polished and has a flatness over itsentire surface of 0.001 inch or better. Smaller tables 36, 37 aremounted on the base table 34 supporting the optical and scanningcomponents of the system. Table 36 is sometimes referred to as theoptical table, and it comprises a horizontally extending slab of granitesupported by steel frame members 38 in the form of I-beams. Table 37 issometimes referred to as the scanning table, and it also comprises ahorizontally extending slab of granite supported by steel side frames39. Thus, tables 36, 37 are rigidly supported above the base table instationary positions.

A carriage assembly 41 is mounted on the base table for holding theoutput medium on which the image of the artwork is to be formed. Thisassembly includes a guide rail 42 which is affixed to the upper surfaceof table 31, and extends longitudinally of that table and beneathscanning table 37. In the preferred embodiment, the guide rail is alsofabricated of granite, and a carriage 43 is mounted on the rail formovement in the longitudinal direction. The carriage is driven by amotor 44 and a lead screw 46. A platen 47 having a highly accurate flatsurface 48 is mounted on the carriage for holding the output medium.

A laser 51 for generating a writing beam of coherent radiation ismounted on table 31 to the rear of scanning table 37, as viewed inFIG. 1. In one presently preferred embodiment for exposing diazo andsilver halide film, laser 51 comprises a Coherent, Inc. Model CR-15argon laser. This laser maintains a relatively constant power outputnotwithstanding variations in the operating power supplied to the laser.

The output of laser 51 is directed to an acousto-optical modulator 52 ofsuitable known design for varying the intensity of the writing beam inaccordance with input signals applied thereto. From the modulator, thewriting beam passes to a turning mirror (not shown) mounted on the basetable and is directed upwardly through an aperture 53 in optical table36 to a second turning mirror 54 which is mounted on that table. Frommirror 54 the beam is directed in a horizontal direction and passesthrough an ultraviolet attenuator/filter 56. The beam then passes to aspherical mirror 57 and then to a parabolic mirror 58 where it isfocused to provide an ultimate spot size no greater than 0.001 inch.From mirror 58, the beam is directed to a dichroic beam combiner 59where it is combined with a reference beam and directed along a commonpath 61 toward the scanning assembly.

A HeNe laser 62 is mounted on table 36 to generate the reference beam.The beam produced by laser 62 is directed horizontally through anobjective lens 63 and a beam expander 64 to a turning mirror 66 whichdirects the beam to dichroic mirror 59 for combination with the writingbeam.

The combined writing and reference beams from dichroic mirror 59 impingeupon a piezoelectric mirror 69 which is mounted on the underside ofscanning table 37. A suitable mirror for this purpose is a Burleigh Inc.Model PZ-90 mirror assembly. This assembly comprises a mirror mounted onthree piezoelectric stacks or tubes spaced 120° apart. By applyingsuitable control voltages to the piezoelectric stacks, the mirror can betilted to any desired angle within its range. In the invention, thepiezoelectric mirror is oriented with two of the stacks aligned with ahorizontal axis 71 which is perpendicular to beam path 61 and to thedirection of travel of carriage 43. These two stacks are maintained at areference potential, and a control signal is applied to the third stackto control the angle of tilt about axis 71.

From piezoelectric mirror 69, the combined beam is directed to a scanner72 of the type described in detail in copending application Ser. No.941,763, filed Sept. 13, 1978, and assigned to the assignee herein.Briefly, this scanner comprises a pyramidal mirror 73 and a roof mirrordoublet comprising flat mirrors 74, 76. The pyramidal mirror has threeflat axially inclined reflective facets and is rotated about its axis ata relatively high speed by a drive motor 77. The scanner is mounted ontable 37 in a stationary position on the upper side of that table. Thebeam from mirror 69 impinges upon one of the rotating facets and isreflected toward mirror 74. This mirror reflects the beam to mirror 76which reflects it back to the same rotating facet from which it wasreflected initially. The beam is then reflected out of the scanner to aflat field lens 79 which is mounted in a stationary position on theupper surface of the scanning table. The beam emerging from scanner 72and lens 79 sweeps horizontally from side to side, with substantially noangular displacement in the vertical direction.

The beam from lens 79 impinges upon a dichroic beam splitter 81 whichreflects the writing beam downwardly toward the carriage assembly andpasses the reference beam to a mirror 82. Beam splitter 81 and mirror 82are mounted in fixed positions by a frame 83 secured to table 37. Therelative positions of the flat field lens, beam splitter and carriageassembly are such that the upper surface of an output medium carried bythe carriage lies in the focal plane of the flat field lens.

From mirror 82, the reference beam is directed to a reference mask 86.As discussed more fully hereinafter, light passing through the referencemask is collected by a fiber optic bundle and sensed by a photodetectoror photo multiplier tube 88 to provide a reference signal correspondingto the position of the writing beam on the output medium. The referencemask is supported in a stationary position on the underside of scanningtable 37 in the focal plane of lens 79.

Referring now to FIGS. 2-6, the carriage assembly is illustrated ingreater detail. Drive motor 44 is mounted in a longitudinally extendingslot 91 of generally rectangular cross section in guide rail 42. Leadscrew 46 is threadedly received in a nut 92 affixed to the carriage. Thecarriage is supported vertically by air bearings 93 which rest upon thehighly polished surface of table 31 to permit substantially frictionlessmovement of the carriage. As illustrated, these bearings includecircular pads 94 connected to the underside of the carriage by posts 96.A similar bearing 93 is provided at each corner of the carriage.Additional air bearings 97 are provided between the carriage and theguide rail. These bearings comprise generally rectangular pads 98 whichface the longitudinally extending vertical side faces 99 of the guiderail. Pads 98 are secured to the lower side of the carriage by mountingbrackets 101, and in the embodiment illustrated, bearings 97 areprovided at both ends of the carriage.

Platen 47 is positioned longitudinally on carriage 43 by means of anadjustable stop 103 and a hydraulic cylinder 104. The platen ispositioned laterally by adjustable stops 106 and hydraulic cylinders107. The stops are mounted on the carriage and positioned to engage theend and side walls of the platen when the platen is in the desiredposition on the carriage. The cylinders are also mounted on the carriageand positioned opposite the stops for pressing the platen against thestops.

Means is also provided for adjusting the position of the platenvertically on the carriage. This means includes hydraulic operatingcylinders 109 mounted on the carriage towards the corners of the platen.Each of the cylinders has an output shaft 111 which is threadedlyreceived in the lower portion of the platen. The bodies of the operatorsare mounted on brackets 112, 113 which are affixed to the carriage.These mounting brackets extend through openings 114, 116 in thecarriage, and brackets 112 include horizontal flanges 117 positionedabove the platen at one end thereof. These flanges carry adjustablestops 118, 119 which engage the upper surface of the platen to positionit vertically. At the other end of the platen, similar adjustable stops121, 122 are mounted on a mounting bracket 123 which is affixed to thecarriage. The stops are arranged in pairs, with stops 119, 122 beingpositioned inwardly of stops 118, 121. The inner stops are carried bydetachable bars 124, 126, which are removably mounted on the inner edgesof flanges 117 and bracket 123 by suitable means such as screws andpositioning dowel pins (not shown). In the preferred embodiment, stops118-119, 121-122 comprise micrometers which permit the platen to beaccurately aligned to position the upper surface of the output medium inthe focal plane of the flat field lens. The paired stops permitadjustments for media of different thicknesses, e.g., 0.007 inch filmand 0.250 inch glass. The removable inner stops permit the system toaccommodate media of different lengths.

Platen 47 is a generally hollow structure having a top wall 131, abottom wall 132, end walls 133 and side walls 134. Pin holes 136 extendthrough the top wall, as illustrated in the enlarged area of FIG. 3, andthe output medium is held flat against the top surface of the platen bya vacuum applied through these holes.

Means is provided for accurately positioning the output medium on theplaten and maintaining this position notwithstanding changes intemperature. This means includes registration pins 137 carried by Invarbars 138 which are affixed to the upper wall of the platen by pins 139at the centerline of the scanning system. The bars are located below thetop wall of the platen, and the registration pins extend throughopenings 141 of slightly greater diameter than the pins themselves. TheInvar bars have a substantially zero coefficient of thermal expansion atthe normal operating temperatures of the system. Therefore, thepositions of the registration pins remain accurately fixed relative tothe vertical centerline of the scanning system even though the platenitself may undergo thermal expansion or contraction. Likewise, an outputmedium having openings engaged by the registration pins is held in arelatively fixed position.

Referring now to FIG. 7, data defining the artwork to be produced isstored on a magnetic tape. In the preferred embodiment, the data isgenerated by scanning input copy, but it can also be generatedelectronically if desired. In either case, the data is spatially codedin that it represents elements located at predetermined spatialpositions in the image. Rather than storing a separate bit for eachelement of the image, the data is compressed and stored in a compressedformat. From the tape, designated 151 in FIG. 7, the data is read intothe memory of a computer 152 such as PDP-11. Terminal 26 is connected tothe computer, and from the computer the data is delivered to adecompressor 153 which converts the compressed data to a serial formatcontaining one bit for each element of the image.

In addition to the image data, the decompressed data includes a 16-bitpreamble containing status control and synchronizing information at thebeginning of each scan line. The system includes a preamble comparator154 which receives the data from decompressor 153, checks the preambleand delivers the image data to buffers 156, 157. The preamble comparatoralso delivers a SYNC signal to a frame reference generator 158. Thisgenerator delivers SEL BUF and SEL BUF signals to buffers 156, 157 tocontrol the buffers so that data can be read into one of them while itis being read out of the other. The outputs of the data buffers areconnected to the inputs of a multiplexer 159, and the SEL BUF signal isapplied to the SELECT input of the multiplexer to determine which bufferthe data will be read out of. The output of multiplexer 159 is appliedto the input of modulator 52 whereby the writing beam is modulated inaccordance with the image data.

The delivery of data from the decompressor to the preamble comparatorand to the data buffers is controlled by a free running master clock 161operating at a suitable frequency such as 4.5 MHz. The data is clockedout of the data buffers and delivered to the modulator under the controlof a reference clock signal which corresponds to the position of thewriting beam on the output medium. The reference clock signal isgenerated by a reference clock generator 162 which receives thereference signal from photomultiplier tube 88. The reference signal isproduced by light from the reference beam passing through the referencemask and being detected by the photomultiplier tube. Since the referencebeam and the writing beam are directed along a common path and deflectedtogether by the scanner, the reference signal is closely synchronized inposition with the writing beam. Each pulse of the reference signalcorresponds to the predetermined distance (e.g., 0.003 inch) on theoutput medium. The reference clock generator increases the rate of thereference signal to provide a reference clock pulse for each 0.001 inchof writing beam travel.

As illustrated in FIG. 8, reference mask 86 comprises a reticle 164having a plurality of opaque areas 166 and transmissive areas 167alternately arrayed along the length of the mask. The opaque areasconsist of opaque bars which are spaced apart on a transparent medium toform the transmissive areas. The lens 79 and the dichroic beam splitter81 have been found to have chromatic aberrations which cause thereference beam to be deflected somewhat farther than the writing beamtoward the ends of the scan line. The magnitude of the error is on theorder of 0.001 inch at the ends of the scan. To compensate for thiserror, the transmissive areas are arranged in a non-linear fashion onthe reticle. These areas are of a uniform width throughout the array,but the spacing between them increases toward the ends of the array. Inother words, the opaque bars are wider toward the sides of the reticlethan in the center, as illustrated in somewhat exaggerated fashion inFIG. 8. This correction assures a precise relationship between thereference clock signal and the position of the writing beamnotwithstanding the chromatic aberrations of the lens and the beamsplitter.

It has also been found that transmission delays in both the electricalcircuitry and the optical elements can result in a positional error inthe formation of the image on the output medium. This error arisesbecauses the reference clock signal is generated when the writing beamis in the position where a given data bit is to be imaged. By the timethe reference clock signal reaches the data buffers and multiplexer, thedata reaches the modulator, and the modulated beam reaches the outputmedium, the beam will have moved from the position where it was when theclock pulse was generated. The effect of this error is compounded bynon-linearities in the velocity of the beams across the scan line. Asdiscussed more fully hereinafter, the beam velocity increases toward thecenter of each scan and decreases toward the ends. Thus, the relativelyconstant transmission delay causes a greater positional error in theimage toward the center of the scan than toward the ends. The error isavoided by generating the reference clock pulse in such manner that itappears to occur prior to the time the beam reaches the point where thedata bit is to be imaged. The output media is shifted to compensate forthe error at the minimum beam velocity, and a previously generated clockpulse is shifted forward in phase to compensate for the variations invelocity. Means for effecting this phase shift is included in thereference clock generator.

As illustrated in FIG. 9, the reference clock generator includes a phaselocked loop comprising a phase detector 171, a voltage controlledoscillator 172, a divide-by-N counter 173, and a delay network 174. Thereference signal from photomultiplier tube 88 is applied to one input ofthe phase comparator, and the output of the phase comparator isconnected to the input of the voltage controlled oscillator.

The output of the oscillator is connected to the input of counter 173,and the output of the counter is connected to the input of the delaycircuit. The output of the delay circuit is connected to the secondinput of the phase comparator. The reference clock signal appears at theoutput of the voltage controlled oscillator. The capacity of counter 173is chosen in accordance with the multiplication to be provided by thecircuit, and the reference clock rate is N times the rate of the inputsignal from the photomultiplier tube. The amount of forward phase shiftis determined by the amount of delay in the loop, and in one presentlypreferred embodiment, a delay on the order of 0.5-0.7 microsecondprovides a phase shift on the order of 14 radians at a frequency of 4.5MHz. With this phase shift, the reference clock signal appears to occurbefore it actually does, and notwithstanding the electrical and opticaldelays in the system, each data bit is imaged at the proper position.

Means is also provided for adjusting the intensity of the writing beamto maintain a uniform exposure of the output medium across the scan linenotwithstanding variations in the horizontal velocity of the beam. Withscanning mirror 73 rotating at a relatively constant angular velocity,the horizontal velocity of the writing beam tends to decrease toward theends of the scan. To compensate for the increased exposure which wouldotherwise result, means is provided for reducing the intensity of thebeam toward the ends of the scan. This means includes an intensitycorrection circuit 176 connected to the phase locked loop 169 of thereference clock generator at a point where a signal proportional to thehorizontal velocity of the beam is present. Such a signal is found atthe output of phase comparator 171 and is illustrated by waveform 177 inFIG. 9. The intensity correction circuit includes an input capacitor 178and an amplifier 179. The signal at the output of this amplifier is avoltage which decreases toward the sides of the scan, and this voltageis applied to modulator 52 to vary the intensity of the writing beam andmaintain a uniform exposure throughout the scan.

Referring again to FIG. 7, frame reference generator 158 provides aSCANNER SYNC signal which is utilized for synchronizing scanner motor77. The scanner motor is connected in a phase locked loop 182 with aphase comparator 183, a voltage controlled oscillator 184, an amplifier185, and an encoder 186. The SCANNER SYNC signal is applied to one inputof the phases comparator, and the output of the phase comparator isconnected to the input of the voltage controlled oscillator. The outputof the oscillator is connected to the input of the amplifier whichprovides the driving signal for the scanner motor. Encoder 186 is ashaft position encoder which delivers an INDEX signal when mirror 73 isin position to begin a new scan. The INDEX signal is applied to a secondinput of the phase comparator.

The SCANNER SYNC signal is also applied to the input of a phase lockedloop 187, which delivers a signal at a suitable multiple (e.g., 10) ofthe SCANNER SYNC pulse rate, and the output of this circuit is connectedto the clock input of a presettable counter 188. The output of thiscounter is a CARRIAGE CLOCK signal which is synchronized with theSCANNER SYNC signal and utilized to control the movement of thecarriage, the carriage speed being proportional to the frequency of theCARRIAGE CLOCK signal.

As noted above, the system can be utilized with a variety of differentoutput media, and the different output media generally require differentexposure times to form a given image. For example, whereas a silverhalide film might require only 2 minutes for an image, a diazo filmmight require 80 minutes for the same image. In order to provide thedesired range of exposure, the carriage speed is variable and the scanlines are repeated. The carriage speed is controlled by a SLOWNESSsignal which is applied to the present inputs of counter 188. Thissignal causes the CARRIAGE CLOCK signal to be reduced by a slownessfactor N, and each scan is repeated N times. Thus, for a slowness factorof 1, the carriage moves at its maximum speed, and each line is scannedonce. For a slowness factor of 10, the carriage speed is reduced toone-tenth of the maximum speed, and the scan is repeated so that eachline is scanned 10 times with the same data.

Carriage motor 44 is driven in synchronism with the CARRIAGE CLOCKsignal by means of a loop comprising an UP/DOWN counter 189, adigital-to-analog converter 190, an amplifier 191, and an encoder 192.The CARRIAGE CLOCK signal is applied to the UP counting input of thecounter, and the output of this counter is connected to the input of thedigital-to-analog converter. The analog output signal is amplified toprovide the driving signal for the carriage motor. Encoder 192 comprisesa reticle having a linear scale for providing a signal corresponding tothe position of the carriage. This signal is applied to the DOWNcounting input of counter 189.

As illustrated in FIG. 10, means is also provided for synchronizing thedelivery of data with the carriage and scanner positions at the start ofan image. The signal from carriage position encoder 192 is applied tothe input of a presettable counter 193, and a count corresponding to theposition at which the image is to begin is applied to the preset inputsof this counter by thumb wheel switches 194. The output of counter 193is connected to the input of a flip-flop 195 and the output of thisflip-flop is connected to one input of a second flip-flop 196. The INDEXsignal from scanner position encoder 186 is applied to a second input offlip-flop 196. In operation, counter 193 counts the pulses from thecarriage position encoder until the carriage reaches the position set byswitches 194. When this count is reached, flip-flops 195, 196 are set,and the next INDEX signal from scanner position encoder 186 which causesflip-flop 196 to deliver a GO signal to computer 152 to initiate thedelivery of data.

The system also provides the compensation for several other potentialdistortions which might otherwise degrade the quality of the outputimage. These distortions include tilting of the scan lines due to thecontinuous travel of the carriage, blurring of the image when the scanlines are repeated on a moving media, improper vertical positioning ofthe scan lines due to irregular movement of the carriage, and a slightcurvature of the scan lines which is introduced by scanner 72.

FIG. 11 illustrates the tilting of the scan lines which occurs when thecarriage is driven continuously during the scanning process. It isassumed that the carriage is driven in the upward direction, asindicated by arrow 197, and that the scanning progresses from left toright, as viewed in this Figure. If the carriage were stationary, thescan lines would extend straight across the output medium in a directionperpendicular to the carriage travel, as indicated by dotted lines 198.However, when the carriage moves during the scanning process, the scanlines are tilted or inclined, as illustrated by lines 199. The amount oftilt is a function of the relative speeds of the carriage and scanner,and for a line spacing of 0.001 inch, a point located toward the rightside of a scan line is nearly 0.001 inch below where it should be.

FIG. 12 illustrates the slight curvature which is introduced into thescan line by scanner 72. In this figure, the desired straight trace isrepresented by dashed line 201, and the slightly curved trace producedby the scanner is illustrated in exaggerated form by line 202. Theactual trace lies on the desired path at the ends and center of the scanline, but is displaced along a curved path between the center and eachend. For a scan line having a length of 17.8", the maximum displacementis on the order of 0.004-0.006 inch.

Means for compensating for these four distortions is illustrated in FIG.13. This means includes a presettable down-counting counter 206 havingthe scanner INDEX signal applied to its clock input and the SLOWNESSsignal applied to its preset inputs. Thus, this counter functions as adivide-by-N counter which divides the scanner INDEX signal by theslowness factor. The overflow output of counter 206 is connected to theload input of a second presettable downcounting counter 207. Thecarriage position signal is applied to the clock input of counter 207,and the weighted output of this counter is connected to one input of anadder 208. A constant SCAN SPACING signal corresponding to the desiredspacing between the scan lines is applied to a second input of theadder, and the output of the adder is connected to the preset input ofcounter 207. For normal carriage movement (SLOWNESS=1), the count inadder 208 is loaded into counter 207 at the start of each scan line. Asthe carriage travels, the pulses generated by the carriage positionencoder reduce the count in counter 207 and adder 208 until the end ofthe scan line is reached. At the start of the next scan line, the countremaining in the adder is transferred to counter 207, thus raising theoutput of the counter by an amount corresponding to one scan linespacing. Thus, as each scan progresses, the level of the signal at theoutput of counter 207 decreases, and a control signal derived from thissignal is applied to piezoelectric mirror 69 to deflect the writing beamto compensate for this tilt.

If the carriage is not in its proper position, the carriage positionpulses will be displaced accordingly, and the scan line will be shiftedby a corresponding amount to compensate for the carriage position error.

When the carriage is traveling at a slower speed (e.g., SLOWNESS=10),the count from adder 208 is not transferred to counter 207 at the startof each scan line. The transfer occurs at the rate determined by theSLOWNESS signal, e.g., every 10 scan lines for a slowness factor of 10.The output of counter 208 continues to decrease, causing the imageforming beam to scan along the same track or line of the output mediumfor 10 successive scan lines. Thus, slowness blur distortion iseliminated at the same time that correction is provided for tilt andcarriage position errors.

To compensate for the curvature introduced into the scan line by scanner72, the amount of compensation required at each point in the scan isdetermined, and data defining the necessary correction voltages isstored in a read only memory 211. Address signals for the ROM areprovided an address counter 212 which is clocked by the reference clocksignal. Thus, as the scan progresses across the line, the correctionsignal data is read out of the ROM and utilized to provide a correctionsignal for piezoelectric mirror 69.

The outputs of counter 207 and ROM 211 are connected to the inputs ofanother adder 216 which combines the tracking and curved scan correctionsignals to provide a single signal which is converted to an analogsignal by a digital-to-analog converter 217. The analog signal isapplied to the piezoelectric mirror via a mirror driver 218. The mirroris thereby tilted to deflect the beam in such manner that it isdisplaced vertically (i.e., in the direction of carriage travel) tomaintain a straight, evenly spaced, unblurred scan line perpendicular tothe axis of carriage travel.

Operation and use of the system, and therein the method of theinvention, can be summarized as follows. The output medium on which theimage of the artwork is to be formed is placed on platen 47, and thecarriage is returned to its starting position, as illustrated in FIG. 1.Carriage drive motor 44 is then energized, and when the carriage reachesthe position at which the image is to begin, the delivery of data fromthe tape and computer begins. The writing beam from laser 51 ismodulated in accordance with the data from the tape, and the beam isscanned across the output medium to form the desired image. Thereference beam from laser 62 is deflected simultaneously with thewriting beam and directed to reference mask 86 to provide the referenceclock signal which controls the delivery of data to the modulator. Asdiscussed above, compensation is provided for chromatic aberration andother distortions which might otherwise prevent the generation of animage having sufficient accuracy for printed circuit artwork.

The electronic control and data output sequencing is illustrated in thetiming diagram of FIG. 14. In this figure, the decompressed data isrepresented by waveform 221. The SCANNER SYNC pulse (waveform 222) isgenerated in response to the preamble word at the start of each line ofthe data. When the system is properly synchronized, the scanner INDEXpulse (waveform 223) coincides with the SCANNER SYNC pulse, and the SELBUF signal (waveform 224) is synchronized with these two pulses. The SELBUF signal determines which buffer the data is read into and out of, andthe reference clock signal (waveform 226) controls the delivery of datato the modulator from the selected buffer. The CARRIAGE CLOCK signal(waveform 227) is synchronized with the SCANNER SYNC signal and is amultiple thereof, e.g. 10 CARRIAGE CLOCK pulses for each SCANNER SYNCpulse. The image data delivered to the modulator in response to thereference clock signal is represented by waveform 228.

The invention has a number of important features and advantages. Theoptical components are extremely stable and free of vibration because ofthe massive structure of base table and other supporting structure. Avariety of output media is readily accommodated, and the artworkproduced by the system is of a uniform high quality throughout theentire image area. The system is capable of producing line widths andclearances as small as 0.002 inch at any point in the image area and atany pattern density. Features and clearances in the output image areaare within 0.001 inch of the size defined by the input data. For a lineparallel or perpendicular to the scan direction, the edge irregularityor resolution is less than ±0.0005 inch, as determined by measuring thelaterial displacement between the innermost and outermost points on theline.

It is apparent from the foregoing that a new and improved system andmethod for producing artwork for printed circuit boards have beenprovided. While only certain presently preferred embodiments have beendescribed herein, as will be apparent to those familiar with the art,certain changes and modifications can be made without departing from thescope of the invention as defined by the following claims.

What is claimed is:
 1. In a system for forming an image of printedcircuit artwork on a generally planar output medium: laser means forproviding a beam of coherent radiation, means for modulating the beam inaccordance with the image to be formed, means for deflecting the beam toeffect scanning of the output medium, means for focusing the scanningbeam at an image plane, a carriage movable in a fixed plane generallyparallel to the image plane, a platen mounted on the carriage and havinga relatively large planar surface for holding the output medium, meansfor moving the platen between advanced and retracted positions relativeto the carriage and the image plane, and stop means carried by thecarriage for abutting engagement with the planar surface of the platento position the output medium carried by the platen in the image planewhen the platen is in the advanced position.
 2. The system of claim 1wherein the stop means is adjustable.
 3. In a system for forming animage on an output medium: means for providing a beam of coherentradiation, means for modulating the beam with data representative of theimage to be formed, a platen for holding the output medium, means fordirecting the beam toward the platen and deflecting the beam to effectscanning of the output medium, first and second bars of a materialhaving a substantially zero coefficient of thermal expansion at normaloperating temperatures affixed to the platen at a reference location,and registration pins carried by the bars and projecting from thesurface of the platen for engaging registration holes in the outputmedium to hold said medium in a constant position relative to thereference, location notwithstanding dimensional changes in the platendue to changes in temperature.
 4. The system of claim 3 wherein theplaten has a generally planar top wall, the bars are fixed beneath thetop wall, and the registration pins extend through openings in the topwall of greater diameter than the pins.
 5. In a system for forming animage of printed circuit artwork on a generally planar output medium:laser means for providing a beam of coherent radiation, means formodulating the beam in accordance with the image to be formed, means fordeflecting the beam to effect scanning of the output medium, means forfocusing the scanning beam at an image plane, a carriage movable in afixed plane generally parallel to the image plane, a platen mounted onthe carriage and having a relatively large planar surface for holdingthe output medium, and means for holding the platen in first and secondpositions on the carriage so that output media of different thicknesseson the planar surface can be positioned in the image plane, said meansfor holding the platen comprising a plurality of operating cylinders formoving the platen between raised and lowered positions on the carriageand stop means carried by the carriage for abutting engagement with theplanar surface of the platen to define the raised position.